On Fri 28-05-21 12:39:54, Feng Tang wrote: > On Thu, May 27, 2021 at 05:34:56PM +0200, Michal Hocko wrote: > > On Thu 27-05-21 21:34:36, Feng Tang wrote: > > > On Thu, May 27, 2021 at 02:26:24PM +0200, Michal Hocko wrote: > > > > On Thu 27-05-21 20:10:41, Feng Tang wrote: > > > > > On Thu, May 27, 2021 at 10:20:08AM +0200, Michal Hocko wrote: > > > > > > On Wed 26-05-21 13:01:42, Feng Tang wrote: > > > > > > > Now the only remaining case of a real 'local' policy faked by > > > > > > > 'prefer' policy plus MPOL_F_LOCAL bit is: > > > > > > > > > > > > > > A valid 'prefer' policy with a valid 'preferred' node is 'rebind' > > > > > > > to a nodemask which doesn't contains the 'preferred' node, then it > > > > > > > will handle allocation with 'local' policy. > > > > > > > > > > > > > > Add a new 'MPOL_F_LOCAL_TEMP' bit for this case, and kill the > > > > > > > MPOL_F_LOCAL bit, which could simplify the code much. > > > > > > > > > > > > As I've pointed out in the reply to the previous patch. It would have > > > > > > been much better if most of the MPOL_F_LOCAL usage was gone by this > > > > > > patch. > > > > > > > > > > > > I also dislike a new MPOL_F_LOCAL_TEMP. This smells like sneaking the > > > > > > hack back in after you have painstakingly removed it. So this looks like > > > > > > a step backwards to me. I also do not understand why do we need the > > > > > > rebind callback for local policy at all. There is no node mask for local > > > > > > so what is going on here? > > > > > > > > > > This is the special case 4 for 'perfer' policy with MPOL_F_STATIC_NODES > > > > > flag set, say it prefer node 1, when it is later 'refind' to a new > > > > > nodemask node 2-3, according to current code it will be add the > > > > > MPOL_F_LOCAL bit and performs 'local' policy acctually. And in future > > > > > it is 'rebind' again with a nodemask 1-2, it will be restored back > > > > > to 'prefer' policy with preferred node 1. > > > > > > > > Honestly I still do not follow the actual problem. > > > > > > I was confused too, and don't know the original thought behind it. This > > > case 4 was just imagined by reading the code. > > > > > > > A preferred node is a > > > > _hint_. If you rebind the task to a different cpuset then why should we > > > > actually care? The allocator will fallback to the closest node according > > > > to the distance metric. Maybe the original code was trying to handle > > > > that in some way but I really do fail to understand that code and I > > > > strongly suspect it is more likely to overengineered rather than backed > > > > by a real usecase. I might be wrong here but then this is an excellent > > > > opportunity to clarify all those subtleties. > > > > > > From the code, the original special handling may be needed in 3 cases: > > > get_policy_nodemask() > > > policy_node() > > > mempolicy_slab_node() > > > to not return the preset prefer_nid. > > > > I am sorry but I do not follow. What is actually wrong if the preferred > > node is outside of the cpuset nodemask? > > Sorry, I didn't make it clear. With current code logic, it will perform > as 'local' policy, but its mode is kept as 'prefer', so the code still > has these tricky bit checking when these APIs are called for this policy. > I agree with you that these ping-pong rebind() may be over engineering, > so for this case can we just change the policy from 'prefer' to 'local', > and drop the tricky bit manipulation, as the 'prefer' is just a hint, > if these rebind misses the target node, there is no need to stick with > the 'prefer' policy? Again. I really do not understand why we should rebind or mark as local anything here. Is this a documented/expected behavior? What if somebody just changes the cpuset to include the preferred node again. Is it expected to have local preference now? I can see you have posted a newer version which I haven't seen yet but this is really better to get resolved before building up more on top. And let me be explicit. I do believe that rebinding preferred policy is just bogus and it should be dropped altogether on the ground that a preference is a mere hint from userspace where to start the allocation. Unless I am missing something cpusets will be always authoritative for the final placement. The preferred node just acts as a starting point and it should be really preserved when cpusets changes. Otherwise we have a very subtle behavior corner cases. -- Michal Hocko SUSE Labs
